1. Field of the Invention
The present invention refers to a method and a device for measuring concentrations preferably in pulp suspensions.
2. Description of the Related Art
Better control over processes in manufacturing industry is one solution in order to obtain a better productivity, better and new qualities and less influence on the surroundings. Therefore measuring on-line is absolutely a necessity. The measurement shows the connections between the process, the raw material and the finished product and gives an increased understanding for the dynamic of the process and with that a basis for taking up more effective control strategies. Measuring on-line can be done in different ways depending on the measuring method and the measuring sequence required. One method is measuring in-line where the measuring sensor is directly mounted in the measuring medium, e.g. a temperature sensor. However, specific measuring parameters often have to be determined on samples taken out from the measuring medium. One reason for this is that the sample before the measuring procedure e.g. has to be diluted to lower concentrations or be pre-treated in another way, for example pH-adjusting, de-airation, dosing of chemicals in order to initiate certain reactions, which thereafter shall be measured etc.
Within pulp- and paper industry concentration measuring on pulp suspensions is of a central importance. These suspensions often contain not only wood fibres but also different fill agents such as clay, calcium carbonate etc. and various chemicals for example for bleaching of the fibres or flock formation between the different suspended substances. Dependent on the measuring position the concentration can vary between one tenth of a procent and ten procent. In certain measuring positions it is extra important to obtain accurate results on the total concentration of fibres inclusive filler, and also on only filler. An in-line meter cannot always give such an information. This can depend on the state of the suspension and/or on the actual design of the method, which is not suitable to base an in-line sensor upon.
For concentration measuring optical measuring principles are often used, especially when differentiated information concerning the composition of the suspension with respect to different suspended materials, shall be obtained. Optic concentration measuring is among other things sensitive for air bubbles in the suspension and flock formation. Flocks constitute “large particles” of many small and give an incorrect measurement signal information. In situations having among other things the criteries mentioned above a special sample treatment equipment is used in order to obtain a satisfactory results. The equipment normally consists of a measuring vessel having an integrated pump loop, in which the measuring sensor is placed. The principle function is as follows: Via a sampling valve a separate pulp sample is fed to the measuring vessel and is thereafter pumped around a certain time before the measuring is made. The pumping around of the sample before the measuring is needed so that the sample shall be de-aired and de-flocculated. After the measurement the sample is drained from the measuring vessel to the outlet and the next measuring cycle is started in that a new sample is taken. Between a number of measurements the whole sample treatment unit can be cleaned by water distributors and filling up with clean water. Besides a measurement can be done on the clean water in order to zero point calibrate the optical measuring system. At too high concentrations the optical measuring principle does not function satisfactory and there is in this case a risk that the measuring cell is plugged by the pulp. Therefore, the sample, when needed, be diluted with clean water. The dilution is controlled via level guards in the measuring vessel.
The conventional method which is here described for a sample treatment unit in connection with for example an optical concentration determination of pulp suspensions functions on the whole. However, there are evident limitations and/or drawbacks:                For the task a relatively big, expensive and energy requiring pump is needed, which besides must be rotation speed regulated in order to obtain a suitable flow rate for the measurement through the measuring transducer and for control of the deaeration. A too high rotation speed instead can create a vortex formation in the measuring vessel, so that air is sucked into the suspension.        The tightening between the motor axle and the pump housing becomes easily untight and relatively often has to be changed. The untightness means that air can be sucked into the suspension, which shall be measured, which spoils the measurement.        The fibres of the pulp suspension in the pump housing can be broken or in another way be changed, so that the measuring results can be wrong. Furthermore, during high rotation speeds in the pump housing cavitation effects can easily arise, so that disturbing air bubbles are created. Dependent on the structure the pump housing can also be difficult to be deaerated at the time of each new measurement cycle.        A pump based sample treatment unit requires a relatively large measuring vessel. The size of the measuring vessel is directly related to the deaeration and deflocculation time and to that time it takes to homogenize the sample, especially when diluted, which is of great importance for the quality of the measurement. Prolonged times also mean that the total time for a whole measurement sequence is prolonged, so that the measurement frequency is too low in order to receive a good control information for an effective process control.        